Coupling Nuts

Coupling Nut Applications

Coupling nuts are used to make up long tie rod assemblies from shorter lengths of rod and to facilitate assembly of two fasteners or fittings which each have external threads.

Relevant Standards

Coupling nut dimensions are described by the Industrial Fasteners Institute in standard IFI-128. Chemical and mechanical properties are controlled in conformance with ASTM and SAE nut specifications (ASTM A563, ASTM A194 and SAE J995).

Finishes

Standard finish is plain with either a cold drawn or hot rolled mill finish, depending on the type of bar stock used. Hot dip galvanizing is available as well as zinc electroplating.

Threads

Standard threads are UNC 2B, RH, with other thread series and profiles available by special order. Hot dip galvanized coupling nuts are normally tapped oversize after galvanizing to accommodate the zinc coating on the rod threads. Coupling nuts up to 1-1/2" thread size are tapped all the way through from one end, allowing them to be completely threaded on to one of the members which are to be coupled, a feature that is required for certain assembly situations. Coupling nuts larger than 1-1/2" thread size cannot be tapped all the way through without special extended length taps and are therefore furnished with RH/RH threads (tapped to a depth of L/2 from each end) unless specifically ordered as 'threaded all the way through'.

Coupling nuts can also be threaded with two different thread sizes or profiles. These parts are normally identified as 'reducing coupling nuts'.

Dimensions

Refer to the coupling nut table for dimensions. Some of the sizes listed in this table have an 'Across Flats Size' slightly larger than that called for by IF-128 due to commercial availability of hexagon bar stock. Because of limited availability of hex stock larger than 3-1/2", coupling nuts with thread sizes 2-1/2" and larger are usually made from round bar. These larger parts can be provided with milled wrench flats or a milled hex.

Notes Regarding Strength

Standard coupling nuts meet the chemical and mechanical requirements of ASTM A563 grade A. When high strength rods are specified, coupling nuts produced to meet ASTM A563 grade DH and A194 grade 2H or grade 7 can be provided. These parts will meet all of the chemical and mechanical properties called for in those specifications.

In order to meet the above strength requirements, the rods must be threaded into the coupling nut for a distance equal to or greater than the nominal thread diameter.

The IFI-128 specification describes an optional cross hole located at L/2, (also known as a 'sight hole') which allows visual verification that the threads are fully engaged.